1. Field of the Invention
This invention relates to a method for preparing isocyanic acid.
2. Description of the Prior Art
An existing method for producing isocyanic acid consists of pyrolyzing cyanuric acid or urea. The method suffers because of the high processing temperatures called for and the need to use relatively costly intermediates as starting raw materials. Another commercially unattractive method involves the extended controlled addition of a mineral acid to potassium cyanate.
Although the current uses of isocyanic acid are rather limited, there is however, the potential usage to prepare aliphatic isocyanates which have extensive industrial application. Accordingly, there is a need for an economical way of producing isocyanic acid particularly if it is to be used as an intermediate in preparing a commodity chemical such as the organic isocyanates.
The genesis of the approach to prepare isocyanic acid in accordance with the chemistry concerned herein is to be found in a number of related papers which began to appear in the early part of the last decade. One of the forerunners of these articles is Unland, M. L., J. Phys. Chem. 77, 1952 (1973). The latter reported work was directed to a study of the reductive removal of nitric oxide from auto exhaust through the agency of a noble metal catalytic converter system. The most significant finding of this study was that carbon monoxide and nitric oxide combined at elevated temperatures to form a complex isocyanate species on the surface of the noble metal catalyst.
The first potential industrial application of the above finding is set forth in U.S. Pat. No. 4,174,377. Therein it is taught that a variety of cyanate compounds, including isocyanic acid, can be prepared directly by contacting a noble metal hydrogenation catalyst at an elevated temperature with a feed stream of nitric oxide, carbon monoxide and a source of hydrogen. The present invention represents an improvement over the foregoing prior art particularly in regards how to enhance the yield of isocyanic acid in carrying out the underlying reaction over supported heterogeneous catalyst.